1. Field of the Invention:
The present invention relates to electromechanical filters of the type having a plurality of resonators coupled to each other by couplers to make it readily possible to provide a transmission frequency characteristic with finite-attenuation poles. The invention relates more particularly to an electromechanical filter in which the coupling coefficients given by the couplers differ from each other.
2. Description of the Prior Art:
Electromechanical filters of the type having a plurality of resonators coupled to each other in a chain by couplers have been widely used, for example, in carrier telephone transmission systems. In this application, an electromechanical filter having mechanical attenuation poles is desirable to provide a steep transmission frequency characteristic. One prior art electromechanical filter is disclosed in "A Twin Tee Multimode Mechanical Filter"published in Proceedings of The IEEE, Vol. 54, pp. 1961-1962, of December 1966. This electromechanical filter is such that a symmetrical lattice circuit is transformed into a symmetrical parallel tee ladder circuit, which is then constituted of dual-mode resonators. This type of electromechanical filter is advantageous in that the number of resonators used can be half that required for the ordinary type which uses single-mode resonators, because the former employs dual-mode resonators. However, because of the dual-mode structure, this type of electromechanical filter has limited applications; the vibration mode of resonators is confined, for example, to the bending-mode vibration of disk resonators. Furthermore, design freedom is limited because the electromechanical filter is constituted essentially of symmetrical circuits. The limited design freedom of the prior art filter will be described below in more detail.
The symmetry of a filter circuit depends on the transmission frequency characteristics of the filter circuit, as well as on the manner of design for realizing such characteristics in the filter circuit. It depends also on whether it is the whole or part of the filter circuit where "symmetry" applies. As for the symmetry depending on the transmission frequency characteristics and on the manner of design, a symmetrical filter circuit is obtained when, for example, the passband Chebyshev characteristic is designed by the operating insertion-loss design technique, while an asymmetrical filter circuit is obtained when the passband Chebyshev characteristic is designed by the voltage insertion-loss design technique.
As for the symmetry of the entire filter circuit, symmetry is obtained for the entire circuit when the passband maximally flat characteristic or the passband Chebyshev characteristic is designed by the operating insertion-loss design technique. While, when other characteristics such as flat delay characteristic and linear phase characteristic are designed or when the passband Chebyshev characteristic is designed by the voltage insertion-loss design technique, symmetry is not available in the entire filter circuit, and therefore it is impossible to realize these characteristics with the prior art mechanical filter.
The symmetry in part of the filter circuit will be described below. In the case of flat delay characteristic, the entire filter circuit is not symmetrical and hence part of the circuit is not symmetrical. In the case of designing Chebyshev characteristic or the maximally flat characteristic by the operating insertion-loss design technique, the entire filter circuit is symmetrical. Therefore, when this circuit is expressed in terms of a lattice circuit, and calculation is repeated so that the common part of the arm impedance is extracted outside, then a symmetrical lattice circuit can be obtained in the center of the filter circuit. As a result, all the finite-attenuation poles are concentrated in the center lattice circuit. Therefore, when the number of finite-attenuation poles is large, the insertion-loss becomes inevitably high in the center lattice circuit, with the result that the sensitivity of the elements in the attenuation region becomes high and the productivity of the electromechanical filter is lowered.